Why are silicon and oxygen the main elements in the Earth's crust?

To start off, Silicon and oxygen are among the most abundant elements in the universe: oxygen is the third most common element and silicon is the seventh. They readily combine to form silicon dioxide (silica), making it one of the most common solid materials in the universe. Additionally, silica is less dense than many other abundant substances within Earth. Therefore when Earth formed, the more silica-rich materials tended to float toward the surface.